This shared instrument proposal is for purchase of a hybrid dual-head microSPECT X-ray CT imaging system (X-SPECT Small Animal SPECT System with Dual Gamma Detectors and X-ray CT Subsystem, Gamma Medica, Inc.), with a goal of radionuclide targeted molecular imaging in small animals. The proposed state-of-the-art SPECT/CT imaging system will support the needs of multiple current NIH funded investigators at Yale University School of Medicine, from the Departments of Medicine, Diagnostic Radiology, Laboratory Medicine, and Psychiatry. The user group works on a variety of projects, which would utilize different features of the proposed equipment. The proposed system will facilitate high resolution planar and microSPECT imaging and reconstruction. The combined SPECT and X-ray CT unit provides inherent registration of functional SPECT images with anatomical CT images for the purpose of image quantification. The CT images allow for correction of attenuation and partial volume errors associated with SPECT imaging, permitting absolute quantification of radiotracer uptake. This system feature will be critical for the development of a program in targeted molecular imaging. Using this system, targeted radiotracers can be evaluated in vivo for biodistribution, specific organ uptake and clearance kinetics. The fusion of SPECT and CT images will enhance analysis of acquired dynamic data, by facilitating region of interest (ROI) identification in generally lower resolution SPECT images. The acquisition of registered SPECT and CT images should also help align SPECT images acquired at different experimental time points. The registration of SPECT and CT data will facilitate correlation of the lower resolution SPECT images with independently acquired MR images. The merging of SPECT and CT data will also facilitate MIRD calculations for estimation of absorbed radiation dose. Thus, the proposed system would meet all of the needs of the investigators of this shared instrumentation grant, allowing for high resolution imaging in small animals and absolute quantification of radiotracer uptake.